In producing a single crystal by the Czochralski method (hereinafter, referred to as "the CZ method"), a single crystal is pulled in an atmosphere inside of a chamber which is maintained at around an atmospheric pressure, for example, a slight pressure of tens to hundreds of millimeters in height by a column of water (hereinafter, referred to as "the atmospheric pressure method") or in an atmosphere inside of a chamber which is maintained under vacuum or at a reduced pressure by a vacuum pump (hereinafter, referred to as "the reduced pressure method").
In recent years, a water sealing bubbler has been used in an exhaust system in both of the above-mentioned pulling methods.
For example, a typical conventional apparatus for producing a single crystal by the atmospheric pressure method is shown in FIG. 4. In the atmospheric pressure method, before the start of pulling a single crystal, a chamber 1 is evacuated by removing air therein by a vacuum pump 2, and then a valve 3 for the vacuum pump 2 is closed, and after that a valve 4 for an argon gas is opened and the chamber 1 is filled with an argon gas. When the pressure inside of the chamber 1 reaches at a slight pressure of tens of millimeters in height by a column of water, a main valve 16 is opened to discharge the argon gas from the chamber 1. At this time, the argon gas is discharged through a water sealing bubbler 5 which holds sealing water 10 in order to maintain the pressure inside of the chamber 1 at a defined pressure (a pressure of tens to hundreds of millimeters in height by a column of water) and to prevent the air from flowing backward into the chamber 1. The pressure inside of the chamber 1 is dependent on the dipped length L of a pipe 6 which is inserted in the sealing water 10 in the bubbler 5.
The pulling of a single crystal is carried out while maintaining such a slight pressure. In order to take out the produced single crystal or exchange a single crystal seed with another one, a pulling chamber 7 must be opened after closing an isolation valve 9. At this time, the flow of the argon gas is changed so as to enter a main chamber 8 below the isolation valve 9 and the inside of the main chamber 8 is maintained at the slight pressure. When such a procedure as taking out the crystal, etc. is finished and a procedure of pulling a single crystal is started again, the pulling chamber 7 is evacuated by removing air therein by the vacuum pump 2 and then is filled with an argon gas. After the inside of the pulling chamber 7 is replaced with an argon gas, the isolation valve 9 is opened and then a procedure of pulling a single crystal is started again.
If by some accident a leakage occurs in the isolation valve 9 in a procedure of evacuating the pulling chamber 7, the inside of the main chamber 8 is also evacuated and the sealing water 10 in the bubbler 5 flows backward into the main chamber 8 through the pipe 6. When such an accident occurs, the water which has flowed backward into the main chamber 8 is evaporated in a moment because the main chamber 8 has a high temperature, which invites danger of steam explosion.
Regarding the reduced pressure method, a vacuum pump used in the conventional reduced pressure method is, for example, an oil-sealed rotary vacuum pump or a water ring type vacuum pump, which have the following disadvantages, respectively:
Since the oil-sealed rotary vacuum pump employs oil, oil mist is contained in an exhaust gas. The exhaust gas is discharged in the air, which leads to air pollution. Therefore, in case of using the oil-sealed rotary vacuum pump, an equipment for treatment of the exhaust gas in a large scale is necessary, which causes an increase in cost, and it is not possible to recover the exhaust gas and recycle the argon gas therein due to the oil mist. On the other hand, the water ring type vacuum pump requires a large amount of water and consumes a high electric power, which also causes an increase in cost.
Accordingly, a dry vacuum pump has been employed in recent years. When the dry vacuum pump is used, the above-mentioned disadvantages of the oil-sealed rotary vacuum pump or the water ring type vacuum pump in the conventional apparatus for producing a single crystal by the reduced pressure method can be avoided. However, when a single crystal is pulled by the CZ method by using the dry vacuum pump, the exhaust gas is discharged in a dry condition, which invites danger of dust explosion in cases where pulverized reaction products included in the exhaust gas, for example, SiO or SiO.sub.2 when a silicon single crystal is pulled, have been deposited on the inside wall of the exhaust pipe. Therefore, it is necessary to install a water sealing bubbler, for example, on the downstream side of the dry vacuum pump to make the exhaust gas moist.
FIG. 5 is a schematic illustration of a conventional apparatus for producing a single crystal by the reduced pressure method by using a dry vacuum pump. In such a conventional apparatus for producing a single crystal by using a dry vacuum pump 11 as shown in FIG. 5, when the dry vacuum pump 11 is stopped in pulling a single crystal in case of emergency, for example, a problem of the pump or a power failure, the sealing water 10 in the bubbler 5 flows backward through the pipe 6 into the chamber 1 because the inside of the chamber 1 is in vacuum or at a reduced pressure, and the backward flowing water is evaporated in a moment in the chamber 1 having a high temperature, which invites danger of steam explosion, similarly to in the atmospheric pressure method.
In order to deal with the above mentioned problems, in a conventional apparatus for producing a single crystal in which a dry vacuum pump is used, some way of preventing the sealing water from flowing backward is used, for example, a check valve 12 is installed on the upstream side of the bubbler 5. However, pulverized reaction products, for example, SiO or SiO.sub.2 when a silicon single crystal is pulled, are included in the exhaust gas by the CZ method, as described above. This leads to incompleteness of the sealing effect of the check valve 12 which results in a leak, and as a result, it is not possible to completely prevent the sealing water from flowing backward.
Apparatuses for producing a single crystal become larger with a single crystal material such as a semiconductor single crystal rod being larger in diameter. Once an accident such as steam explosion happens, the manufactures of single crystals will suffer a deathblow. The present invention was made in view of foregoing problems, and therefore an object of the present invention is to provide an apparatus for producing a single crystal wherein an exhaust system has a water sealing bubbler, which is capable of avoiding danger of steam explosion by taking an easy and reliable way of preventing the sealing water in the bubbler from flowing backward.